This invention relates generally to corrosion-resistant coatings and more specifically to the formation of thin, clear, corrosion-resistant coatings on aluminum and black plate steel containers by crosslinking water soluble acrylic polymers, which contain carboxyl groups, with divalent metal ions.
Metals such as aluminum and its alloys are treated with materials to form coatings which provide corrosion and tarnish resistance to the metal surface. This is especially important in forming metal food and beverage containers. During the pasteurization process, the heat will cause the containers to discolor if they are not protected with a conversion coating. The coatings also act to improve the adherence of overlying coatings such as are formed from paints, inks, and lacquers. In the past, these coatings have been formed from acidic solutions of heavy metals. Currently aqueous coating solutions containing compounds of titanium, zirconium and/or hafnium, a strong mineral acid and fluorides are used. These solutions have the advantage of avoiding the use of materials such as chromates, ferricyanide or tannins but still employ fluorides. Many acidic coating systems have the disadvantage of producing overetched surfaces when the process is not carefully controlled. The container must also be carefully rinsed after coating and before drying. Fluoride-free alkaline container coating systems have also been disclosed which include silicate materials.
A problem occurs when preparing untinned steel containers for the application of protective coatings, because flash rust occurs during drying after the cleaning process.
Aqueous, alkaline solutions of acrylic resins which are crosslinked by divalent metal ions are known for use in forming water-borne, corrosion-resistant coatings but these coating compositions are unsuitable for forming clear, thin, uniform corrosion-resistant coatings on containers which will adhere to container coating lacquers.
We have now found an improved acrylic resin alkaline coating solution and process which is free of toxic materials, which permits the coated containers to be dried directly without rinsing, and which provides excellent corrosion protection and adhesion to coating lacquers.